Reprinted  from  the  Journal  of  Geology,  Vol.  XIV,  No.  4,  May-June,  1906 


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CONDITIONS'  OF  FOSSILIZATION 


J.  CULVER  HARTZELL 


1 


PRINTED  AT  THE  UNIVERSITY  OF  CHICAGO  PRESS 


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CONDITIONS  OF  FOSSILIZATION- 


5&b 


J.  CULVER  HARTZELL 
University  of  the  Pacific,  San  Jose,  Cal. 

Introduction. 

Superficial  Consideration  of  Fossils. 

Conditions  in  Which  Fossils  Occur. 

Difficulties  Encountered. 

Authors’  Definitions  of  Molds  and  Casts. 

Definitions  of  Molds  and  Casts  Followed. 

Definitions  of  “Original,”  “Mold,”  and  “Cast”  Proposed. 

Lithological  Character  of  Formations,  as  Affecting  the  Preservation  of  Inverte¬ 
brates. 

General  Character  of  Invertebrate  Skeletons. 

General  Mineral  Character  of  Living  Invertebrates. 

Replacing  Minerals. 

Horizon,  Locality,  and  Lithological  Character  of  Formations  Studied. 

Minerals  Replacing  Original  Minerals  Secreted  by  Invertebrates. 

INTRODUCTION 

In  the  study  of  a  fossil  the  first  important  point  to  determine  is  its 
state  of  preservation — whether  the  fossil  under  consideration  be 
the  original ,  a  cast  of  the  original,  or  a  mold  of  the  original. 

By  not  observing  this  precaution,  errors  have  been  made,  and 
will  continue  to  be  made  unless  it  be  definitely  understood  what 
is  meant  by  these  terms  and  the  conditions  they  represent  in  fossils. 

SUPERFICIAL  CONSIDERATION  OF  FOSSILS 

A  superficial  consideration  of  a  fossil  is  often  apt  to  lead  to  a 
misinterpretation  of  its  condition  of  preservation,  for  fossils  vary 
in  this  respect.  Molds  may  be  taken  for  casts ,  and  described  as 
exhibiting  the  external  structure  of  the  original. 

1  This  paper  was  largely  prepared  in  1898-99,  under  the  direction  of  the  late 
Professor  Charles  E.  Beecher,  when  the  writer  was  a  graduate  student  at  Yale,  and 
was  submitted  for  the  degree  of  master  of  science.  Its  publication  has  been  delayed 
because  the  writer  wished  to  collect  further  data,  which  he  has  done  in  Europe  and 
America.  Another  paper  is  in  course  of  preparation,  in  which  an  attempt  to  formu¬ 
late  laws  governing  conditions  of  fossilization  will  be  made. 

The  writer  also  wishes  to  take  this  opportunity  to  thank  Professor  S.  L.  Penfield, 
of  Yale,  for  valuable  suggestions  during  the  preparation  of  the  original  manuscript. 

269 


fl32«l 


270 


J.  CULVER  HARTZELL 


We  find  the  same  organisms  preserved  in  all  manner  of  conditions, 
and  only  by  a  careful  comparative  study  of  the  exterior  and  interim* 
markings  are  misinterpretations  to  be  avoided. 

Fossils  of  the  same  species  have  been  referred  to  different  species, 
and  the  same  genera  to  different  genera.  For  example,  Miche- 
linia  clappii ,  Hall,1  was  misinterpreted  by  Edwards  and  Haime, 
and  referred  to  Chonostegites  clappii2 3 4 *  and  also  to  Emmonsia  ( ?) 

cylindrical  and  by 
Billings  to  Haimeophyl- 
lum  ordinatum 4  and  also 
to  Michelinia  inter  mit¬ 
tens.  5  (See  Figs.  1,  2, 

3>  4,  5,  6>  7-)  This 
misinterpretation  was 
due  to  the  peculiar 
ways  in  which  this 
genus  occurs.  At  times 
the  walls  are  coated 
with  silica,  and  then 
Fig.  1. — Chonostegites  clappi.  (After  Edwards  filled  in  with  calcite. 
and  Haime.)  At  times  only  the  silice¬ 

ous  coatings  are  left,  which  give  a  mold  of  the  inner  walls.  Then 
again  we  find  that  they  were  filled  with  calcite,  the  walls  having 
disappeared;  and  in  this  case  we  have  a  solid  mold  instead  of  a 
hollow  one.  At  times  the  form  is  partly  destroyed,  leaving  molds, 
casts,  and  parts  of  the  original  in  the  same  specimen. 

We  find  fossils  of  the  same  species  preserved  (a)  in  their  original 
condition,  ( b )  as  casts,  and  ( c )  as  molds.  In  regard  to  the  first 
condition  little  difficulty  will  present  itself.  The  second  and  third, 
however,  may  lead  to  confusion,  for  they  may  not  exhibit  the  exter¬ 
nal  form. 


1  Hall,  Geology  of  the  State  of  New  York  (1876),  “Illustrations  of  Devonian 
Fossils,  ”  Plate  XVII. 

2  Edwards  and  Haime,  Pal.  Fos.  d.  Ten.  Pal.  (1851),  p.  299,  Plate  XIV,  Figs.  4,  4a. 

3  Ibid. 

4  Billings,  Can.  Jour.  U.  S.,  Vol.  IV  (1859),  p.  139. 

s  Ibid.,  p.  1 13. 


CONDITIONS  OF  FOSSILIZATION 


271 


CONDITIONS  IN  WHICH  FOSSILS  OCCUR 

The  following  illustrations  will  show  the  various  conditions  in 
which  fossils  are  found,  and  they  will  also  serve  to  show  the  necessity 
for  close  observation  and  comparative  study. 

A.  The  original  skeleton  may  be  preserved.  If  there  be  hollows 
or  spaces,  they  may  become  filled 
with  infiltrating  material.  In 
a  case  of  this  kind  little  difficulty 
will  present  itself  in  the  determina¬ 
tion  of  the  fossil. 

B.  The  original  skeleton  may 
be  replaced  by  some  mineral  and 
the  cavities  filled  with  the  same,  or 
some  other  material. 

If  the  skeleton  were  composed 
of  aragonite,  and  were  replaced 
by  calcite,  the  external  form  and 
markings  would  be  preserved;  but 
the  internal  organic  structure  would 
be  lost,  and  hence  not  seen  under  the  microscope.  In  this  case  we 
should  depend  upon  external  markings  for  identification. 

If  a  skeleton  composed  of  calcite 
were  replaced  by  pyrite,  we  could  not 
ascertain  in  thin  section  whether  the 
internal  organic  structure  were  lost  or 
preserved,  because  of  the  opaqueness 
of  the  pyrite,  and  again  we  should 
depend  upon  external  markings.  A 
broken  section,  however,  will  show  on 
the  fractured  surface  the  minutest 
details,  in  many  cases. 

If  the  organism  were  replaced  molec- 
ularly  by  a  mineral  which  transmits  light,  the  internal  organic 
structure  would  be  so  well  preserved  as  to  be  readily  distinguished 
under  the  microscope.  In  a  case  of  this  kind  we  should  have  a 
double  check — the  internal  as  well  as  the  external  structure — and 
its  identification  would  be  doubly  sure. 


Fig.  3. — .Chonostegites 
clap  pi.  (After  Miller.) 


Fig.  2. — Chonostegites  clappi. 
(After  Edwards  and  Haime.) 


■ 


272 


J.  CULVER  HARTZELL 


C.  The  organism  may  disappear  after  its  cavities  or  hollows 
were  filled  by  infiltrating  material.  In  this  case  we  should  have 
only  the  impression  of  the  interior  of  the  original,  and  it  would  be  neces¬ 
sary  to  compare  this  with  the  interior  of  skeletons  already  known. 
This  is  at  times  difficult;  but  it  is  the  only  alternative.  To  make  this 
comparison  it  might  be  necessary  to  take  an  impression,  or  cast, 
from  the  exterior  of  this  filling,  which  cast  would  show  the  markings 
of  the  interior  of  the  original.  Unless  we  could  find  a  fossil  or  a 


Fig.  4. — Michelinia  clappi.  (After  Hall.) 

living  form  showing  the  same,  or  nearly  the  same,  internal  markings, 
we  should  be  baffled  in  any  attempt  to  adjust  it  to  its  zoological 
position,  and  hence  its  identification  would  remain  unsolved  until 
one  of  these  two  conditions  was  satisfied. 

D.  The  hard  parts  of  an  organism  may  leave  only  an  impression 
of  its  exterior  in  the  matrix.  It  will  then  be  necessary  to  take  an 
impression,  or  cast ,  from  this  first  impression,  or  mold,  and  upon 
the  markings  shown  on  this  cast  will  depend  the  identification  of 
the  fossil. 


CONDITIONS  OF  FOSSILIZA  T ION 


273 


E.  The  exterior  of  the  skeleton  may  become  coated  with  some . 
mineral  such  as  silica,  after  which  the  skeleton  may  disappear. 
In  this  case  we  should  have  a  hollow  mold.  It  would  then  be  neces¬ 
sary  to  take  an  impression,  or  cast,  from  this  mold  in  order  to  ascer¬ 
tain  the  external  markings  of  the  original,  and  by  comparing  this 
cast  with  known  forms  we  can  determine  its  identity.  At  times 
the  corallites  in  a  compound  coral  will  become  coated  with  silica, 
and  the  spaces  between  the  corallites  filled  with  calcareous  material 


Fig.  5. — Michelinia  clap  pi.  (After  Hall.) 

the  entire  corallum  having  disappeared,  leaving  a  mass  filled  with 
these  hollow  siliceous  tubes,  the  inner  surfaces  of  which  will  be 
molds  of  the  exteriors  of  the  corallites.  (See  Michelinia ,  pre¬ 
viously  referred  to.) 

We  may  have  the  exterior  coated  with  silica,  the  skeleton  then 
disappearing,  and  the  space  left  filled  with  calcite.  In  a  case  of  this 
kind  we  have  molds  of  the  exterior,  and  it  is  impossible  to  identify 
the  fossil  without  first  dissolving  out  the  calcite,  after  which  the 
procedure  will  be  as  already  described. 


274 


/.  CULVER  HARTZELL 


The  interior  cavities  or  hollows  of  a  coral  may  become  coated 
with  silica,  after  which  the  skeleton  may  disappear,  leaving  molds 
of  the  interior  which  will  have  the  appearance  of  a  sponge.  If 
we  had  a  coating  which  had  been  deposited  upon  the  inner  surface 
of  the  shell,  it  might  be  easily  determined  by  taking  a  cast  for  com¬ 
parison  with  other  shells;  but  in  the  former  case  its  identification 
would  be  extremely  difficult,  for  we  might  not  be  able  to  secure 
casts  from  these  molds.  Even  if  this  could  be  done,  we  might  still 
have  difficulty  in  its  identification,  for  we  should  have  only  casts 
of  the  interior  of  the  cavities  or  hollows  of  the  original  for  com¬ 
parison. 

DIFFICULTIES  ENCOUNTERED 

From  the  foregoing  it  is  plain  that  difficulties  present  themselves 
even  when  it  is  known  that  the  fossil  in  hand  is  the  original ,  a  mold, 
or  a  cast;  but  the  difficulties  increase  if  it  be  not  known  what  the 
condition  of  preservation  is.  Cases  present  themselves  in  which 
it  is  a  very  difficult  matter  to  decide  whether  the  fossil  is  a  cast  or 
a  mold;  but  in  the  majority  of  cases  this  difficulty  is  obviated  by 
close  observation  and  an  understanding  of  the  meaning  of  casts 
and  molds . 

It  is  only  by  a  study  of  casts  and  molds  in  their  various  condi¬ 
tions — found  as  fossils  or  made  in  the  laboratory — that  we  may 
with  a  certain  degree  of  exactness  determine  the  condition  of  pres¬ 
ervation  of  a  fossil.  The  internal  markings  of  some  forms  resemble 
the  external  markings  of  other  forms,  and  it  is  only  in  the  above 
way  that  we  may  be  certain  that  we  are  dealing  with  external  or 
internal  markings. 

There  is  a  wide  difference  between  a  cast  and  a  mold.  Ca  ts 
vary  in  that  some  do  and  some  do  not  show  the  structure  of  the  organ¬ 
ism.  Receptaculites  oweni,  Hall,  from  the  Galena  (Lower  Silu¬ 
rian)  at  McGregor,  Iowa,  represents  the  inner  surface  of  the  skele¬ 
ton,  and  is  a  cast.  (See  Figs,  i  and  8.)  “In  most  specimens 
.  .  .  .  the  remains  consist  of  the  filling  of  the  intermural  space, 
with  casts  of  the  outer  surface  of  the  inner  wall,  the  inner  surface 
of  the  outer  wall,  and  of  the  connecting  tubes.”1  This  is  a  calcite 

1  Bernard,  Principles  of  Paleontology,  Fourteenth  Annual  Report,  New  York 
Geology  (1895),  pp.  89,  90. 


CONDITIONS  OF  FOSSILIZATION 


275 


cast  of  the  interior,  and  has  been  regarded  as  having  the  structure 
of  the  original  organism.  The  conditions  were  evidently  such  as 
to  preclude  its  preservation  in  its  original  condition,  or  in  such  a 
condition  as  to  render  its  determination  certain. 

authors’  definitions  of  molds  and  casts 

Authors  differ  in  their  definitions  of  molds  and  casts.  Some 
make  the  terms  synonymous.  Others  define  them  separately, 
but  are  not  consistent  in 
their  application. 

Darwin1  considers  the 
mold  as  a  matrix,  and 
the  impression  made  by 
an  organism  in  this 
matrix  he  terms  a 
“cast.  ” 

Bernard2  applies  the 
term  “mold”  to  three 
distinct  results :  ( a )  to 
an  impression  made  by 
the  exterior  or  the  in¬ 
terior  of  a  shell;  (b)  to 
molecularly  replaced 
organisms ;  and  (c)  to 
fillings  cf  impressions. 

Gratacap3  applies 

the  term  “cast”  to  (a) 
fillings  which  take  the  Fig.  6 .-MicheUnia  clappi.  (After  Rominger.) 

place  of  organisms,  to  (b)  the  material  filling  the  space  occupied  by 
the  soft  parts.  The  term  “mould”  he  applies  to  impressions  of 
the  exterior  of  an  organism. 

White4  applies  the  term  “mold”  to  impressions  made  by  the 
organism.  To  the  material  filling  this  “mold”  he  applies  the 

1  Darwin,  Geological  Observations ,  Vol.  II,  p.  414. 

2  Bernard,  op.  cit. 

3  Gratacap,  “Fossils  and  Fossilization,  American  Naturalist,  Vols.  XXX,  XXXI, 
pp.  288,  902,  903. 

4  White,  Con.  of  Pres,  of  Inver.  Fos.,  Bull.  U.  S.  G.  &  G.  S.,  Vol.  V,  No.  1,  p.  135. 


276 


J.  CULVER  HARTZELL 


term  “cast.”  He1  also  uses  the  term  “ histometabasis ”  for  the 
condition  which  produces  a  molecular  replacement  or  substitution 
or  paramorphism.2  He  uses  the  term  “fossil  pseudomorphs  ” 
for  the  materials  occupying  cavities  formerly  occupied  by  shells, 
the  occupation  having  taken  place  by  precipitation  due  to  infiltra¬ 
tion.  He  uses  the  term  “fossil  molds”  for  “cavities  in  sedimentary 

rocks  which  were  originally 
occupied  by  fossils,”  and 
says  that  “the  original  sur¬ 
face  features  and  markings 
are  often  minutely  preserved 
in  molds.”  He  also  uses 
the  term  “casts”  for 
“counter-parts  of  fossils,” 
and  also  for  the  material 
which  may  occupy  the 
animal  chamber.  He 
further  speaks  of  making 
“artificial  casts  of  natural 
molds”  in  order  to  get 
“the  original  form  and 
surface  features.” 

Geikie3  applies  the  term 
“mold”  to  impressions 
made  by  the  organism. 
Fig.  7. — Michelinia  clappi.  (After  Rominger.)  To  the  material  filling  this 

“mold”  he  applies  the  term 
“cast.”  He4  also  applies  the  term  “cast”  to  the  material  occupying 
the  animal  chamber. 

Von  Zittel 5  applies  the  term  “mold”  to  impressions.  The  term 
“cast”  he  applies  to  the  material  which  occupies  the  “interior  of  a 
shell  or  hollow  body.” 

1  White,  “Relation  of  Biological  and  Geological  Investigations,  Proceedings  o) 
the  U.  S.  N.  M.,  Vol.  XV,  pp.  264-67. 

2  Dana,  Text-Book  0}  Mineralogy,  (1898),  p.  293. 

3  Geikie,  Text-Book  0}  Geology,  3d  ed.,  p.  651. 

4  Geikie,  Outlines  0}  Field-Geology,  5th  ed.,  p.  78,  Fig.  14. 

5  Von  Zittel,  Text-Book  0}  Palaeontology,  Eastman  translation,  Vol.  I,  Part  1,  p.  2. 


CONDITIONS  OF  FOSSILIZATION 


277 

Woods1  applies  the  term  “mold”  to  (a)  the  impression,  to  ( b )  the 
material  filling  the  space  occupied  by  the  animal.  The  term  “cast” 
he  applies  to  ( a )  the  material  filling  the  space  occupied  by  the 
organism,  and  to  ( b )  the  material  filling  the  internal  cavity  or 
cavities. 

Nicholson  and  Lydekker2  use  the  terms  “meld”  and  “cast” 
interchangeably. 

Lyell3  applies  the  term  “mold”  to  the  matrix  in  which  an  impres¬ 
sion  of  the  exterior  has  been  made.  The  term  “cast”  he  applies 


Fig.  8. — Receptaculites  oweni ,  Hall.  (From  specimens  in  Yale  Museum.) 

to  (a)  the  material  filling  the  interior  of  the  organism,  and  to  (b) 
the  material  filling  the  space  left  by  the  organism. 

Penning4  uses  the  term  “unchanged  fossils”  for  unaltered  shells 
or  valves,  cr  those  which  have  lest  only  the  animal  matter.  “Re¬ 
placed  fessils”  he  applies  to  the  material  which  has  been  substi¬ 
tuted  for  original  material  of  the  shell.  “Internal  cast”  he  uses 
for  “the  impression  or  reversed  facsimile  oj  the  external  jorm  of  the 
organism 5  that  once  filled  the  empty  space”  of  the  shell.  The 
term  “external  cast”  he  uses  for  impressions  made  by  the  exterior 
of  the  shell,  and  says  that  “by  taking  an  artificial  cast  from  the 

1  Woods,  Palaeontology ,  2d  ed.,  pp.  6,  7. 

2  Nicholson  and  Lydekker,  Manual  of  Palaeontology ,  3d  ed.,  Vol.  I,  pp.  5,  6. 

3  Lyell,  Students'  Elements  of  Geology ,  3d  ed.,  pp.  42-46. 

4  Penning,  Text-Book  of  Field-Geology,  2d  ed.,  pp.  208-12,  and  Fig.  29,  p.  211. 

s  The  writer’s  italics. 


278 


J.  CULVER  HARTZELL 


external  impression”  we  obtain  “an  accurate  representation  of 
the  pre-existing  'shell.  ” 

Williams1  is  perplexing  in  his  use  of  the  terms  “mold”  and 
“cast.”  He  says: 

Thus  a  fossil  ....  may  consist  of  the  shell  now  removed,  in  which  case 
it  may  be  the  reverse  or  cavity  over  the  exterior  of  the  shell,  or  ...  .  similar 
impressions  of  the  inner  surface;  or  the  cavity  may  be  again  filled  with  detrital 
matter,  forming  a  cast  of  either  the  inner  or  outer  form  of  the  shell  or  object 
fossilized;  in  the  former  case  it  would  be  called  a  mold;  in  the  latter,  a  cast. 

Schuchert2  applies  the  term  “mold”  to  impressions  of  the  exte¬ 
rior  and  speaks  of  the  mold  as  “preserving  the  exterior  form  and 
ornamentation”  of  the  shell.  He  is  ambiguous  in  his  use  of  the 
term  “cast”  for  he  may  be  referring  to  a  matrix  which  contains 
concave  impressions  (impressions  of  the  exterior  of  a  valve)  or 

i 

convex  impressions  (impressions  of  the  interior  of  a  valve),  or  to 
the  material  which  replaces  a  valve. 

DEFINITIONS  OF  “MOLDS”  AND  “CASTS”  FOLLOWED 

The  definitions  followed  in  this  paper  are  the  following:  A 
mold  is  “a  form  or  model  pattern  of  a  particular  shape,  used  in 
determining  the  shape  of  something  in  a  molten,  plastic,  or  other¬ 
wise  yielding,  state.'”  “In  founding,  a  viold  is  the  form  into  which 
a  fused  metal  is  run  to  obtain  a  cast.  ”3 

The  mold  determines  the  shape  of  the  material  put  in  or  upon  it, 
and  this  material,  when  removed,  will  be  an  exact  duplicate  of  the 
object  from  which  the  mold  was  made.  This  removable  material 
is  termed  a  cast.  The  depressions  in  the  original  object  will  appear 
as  protuberances  in  the  mold,  and  the  protuberances  as  depressions. 
The  cast  will  show  the  depressions  and  protuberances  as  they  appear 
in  the  original. 

The  surface  of  the  original  object  upon  or  around  which  the 
mold  is  made  may  be  either  convex  or  concave.  If  it  be  concave, 
the  mold  will  be  convex,  and  vice  versa.  A  mold  with  a  convex 
surface  is  called  by  some  authors  a  “cast.”  If  the  skeletal  part 

1  Williams,  Geological  Biology  (1895),  p.  79. 

2  Schuchert,  “Directions  for  Collecting  Fossils,”  Part  k,  Bulletin  No.  39,  U.  S. 
N.  M.,  p.  13. 

3  Century  Dictionary ,  “Mold.” 


CONDITIONS  OF  F0SSIL1ZATI0N 


279 


of  an  organism  be  hollow  or  has  a  concavity,  this  space  is  considered 
a  “mold”  from  which  a  “cast”  is  taken.  This  is  obviously  wrong, 
for  in  this  case  the  “mold”  is  the  object ,  and  the  “cast”  the  mold 
from  which  a  cast  may  be  taken,  and  this  cast  will  be  a  duplicate 
of  the  object. 

Then,  again,  an  impression  from  a  convex  object  is  termed  by 
some  a  “cast.”  This  is  also  obviously  wrong,  for  the  “cast”  would 
have  the  markings  of  the  object  in  reverse  order;  hence  it  would 
be  a  mold. 

The  general  concavity  or  convexity  of  a  surface  will  not  deter¬ 
mine  it  as  a  mold  or  a  cast.  Such  determinations  depend  upon 
the  markings  of  that  surface. 

DEFINITIONS  OF  “ORIGINAL,”  “MOLD,”  AND  “CAST”  PROPOSED.. 

In  attempting  to  fix  the  meaning  of  the  terms“ original, ”  “mold,” 
and  “cast,  ”  it  is  hoped  the  following  definitions  will  prove  acceptable, 
especially  the  latter  two: 

I.  The  term  original  is  used 
to  designate  an  organism1  that 
has  not  lost  its  original  structure 
or  composition,  to  any  appreci¬ 
able  extent,  in  the  process  of 
fossilization,  except  the  organic 
matter  which  may  have  filled  the 
interstitial  spaces.  (See  Fig.  9  a 
and  b.) 

II.  The  term  mold  is  used  to 

designate  the  imprint  of  the  ex¬ 
terior  or  interior  of  an  organism.  Fig.  9.— (a  and  b)  originals;  (c)mold 

(a)  If  the  organism  leave  an  im-  of  interior. 

print  of  the  exterior,  this  imprint  is  a  mold  oj  the  exterior.  (See  Fig.  10.) 

( b )  If  the  hollow  organism  become  filled  with  material  this  materia ^ 

1  Strictly  speaking,  a  lifeless  animal  is  not  an  organism;  but  in  common  parlance 
the  lifeless  body  is  an  organism  because  it  is  that  which  at  one  time  functioned.  Like¬ 
wise,  we  speak  of  the  products  of  life  as  organic.  Therefore,  for  lack  of  a  beter  term, 
organism  is  used  to  denote  the  harder  parts  of  animals  which  we  term  fossils,  the 
softer  or  destructible  parts  of  which  have  decayed  and  passed  away.  The  term 
“organism”  can  in  no  sense,  however,  be  applied  to  molds  and  casts,  although 
these  are  fossils  as  much  as  the  unaltered  skeletal  parts  of  animals. 


28o 


J.  CULVER  HARTZELL 


is  a  mold  oj  the  interior.  (See  Figs,  gc,  n,  12.)  (c)  It  follows  from 

the  above  that,  if  the  hollow  organism  become  filled  with  and  im¬ 
bedded  in  material  of  the  same,  or  different,  composition,  and  then 

disappears,  we  have  left  a 
mold  of  the  exterior  in  the 
matrix  and  a  mold  of  the 
interior  in  the  form  of  a 
kernel.  (See  Figs,  gc,  10.) 

III.  The  term  cast  is  used 
to  designate  the  material 
which  takes  the  place  of  the 
original,  whether  by  replace¬ 
ment  due  to  a  molecular 
process  or  to  infiltration.  It 
is  also  used  to  designate  the 
material  occupying  the  mold  made  by  the  exterior  or  interior.  (See 
Figs.  10,  12,  13,  14.) 

If  the  mold  described  under  II  (a)  becomes  filled,  we  have  a  cast 
of  the  exterior.  If  the  mold  described  under  ( b )  becomes  imbedded, 
its  imprint  will  be  a  cast  of  the 
interior.  If  the  space  between 
the  two  molds  described  under 
(c)  becomes  filled,  we  have  a  cast 
of  the  exterior  and  interior,  and 
therefore  an  object  the  same  in 
shape  and  outline  as  the  original. 

If  the  original  be  gradually 
replaced  molecularly  by  some 
mineral,  we  have  a  cast  which 
will  show  its  shape,  outline,  and 
internal  structure. 

From  the  foregoing  it  is  obvious  that  an  original  is  the  organism 
itself;  a  mold,  the  reverse  of  the  original ;  and  a  cast,  the  counter¬ 
part  of  the  original.  The  latter  may  or  may  not  show  the  internal 
organic  structure. 

It  follows,  therefore,  that  the  only  way  one  may  know  whether 
the  markings  on  certain  molds  or  casts  represent  the  exterior  or 


CONDITIONS  OF  FOSSILIZATION 


281 


interior  of  known  forms  is,  as  has  been  previously  said,  to  make 
a  study  of  molds  and  casts,  and  thus  reduce  the  liability  of  mistakes 
to  a  minimum. 

THE  LITHOLOGICAL  CHARACTER  OF  FORMATIONS  AS  AFFECTING  THE 

PRESERVATION  OF  INVERTEBRATES 

The  conversion  of  an  organism  into  a  fossil  depends  upon  the 
character  of  its  skeletal  parts,  the  material  in  which  it  is  buried, 
and  the  material  brought  in,  in  solution,  by  infiltration.  The  material 
of  which  the  skeletal  part  is  composed  varies  in  different  groups, 
being  more  durable  in  some  than  in  others,  and  therefore  plays  an  im¬ 
portant  part  in  the  preservation 
of  the  organism.  The  variation 
in  the  lithological  character  of 
the  material  in  which  the  organ¬ 
ism  is  buried  also  plays  an  im¬ 
portant  part  in  its  preservation. 

Certain  organisms  are  preserved 
as  originals;  others  as  molds 
and  casts,  in  the  same  forma¬ 
tion  and  locality.  In  this  same 
formation,  but  in  a  locality  of 
different  lithological  character, 
those  groups  which  were  lost  under  the  former  condition  may  be 
retained  under  the  latter,  and  vice  versa. 

Apparently  a  law  could  be  formulated  to  the  effect  that  organisms 
of  the  same  mineral  composition  will  be  preserved  in  the  same  man¬ 
ner,  as  originals,  molds,  or  casts.  In  reality,  however,  this  is  not 
true.  Organisms  are  more  completely  preserved  as  originals  in 
limestone;  yet  it  is  in  limestone  that  we  find  the  most  casts  by  molecu¬ 
lar  replacement.  Molds  and  casts  are  very  common  in  sandstones. 
As  limestone  approaches  dolomite,  the  molds  and  casts  increase, 
although  we  also  find  originals.  We  find  molds  in  hematite;  but 
they  are  more  rare  than  in  sandstones. 

The  most  perfect  fossils  are  found  in  sandy  and  clayey  shales. 
The  Niagara  group  at  Waldron,  Ind.,  is  made  up  of  fine  calcareous 
shales  which  are  overlain  by  limestone.  In  these  shales  we  find 


Fig.  12. — Mold  of  the  interior. 


282 


J.  CULVER  HARTZELL 


quantities  of  corals,  bryozoans,  and  crinoids.  The  brachiopods 
are  more  or  less  preserved  as  originals.  The  sponges,  gastropods, 
annelids,  and  crustaceans  are  well  represented.  The  lamellibranchs 
and  cephalopods,  however,  are  absent. 

The  Paleozoic  hexactinellids  occur  in  groups  of  strata  containing 

other  organisms;  but  in  their 
own  particular  beds  the 
absence  of  other  forms  is 
striking.  In  Steuben  county, 
N.  Y.,  the  formation  is  a 
sandstone  which  is  fine-grained 
and  argillaceous,  and  contains 
very  few  crinoids  and  brachio¬ 
pods;  but  Hydnoceras  tube¬ 
rosum  occurs  in  abundance.1 

The  fossils  in  the  sandy 
and  gravelly  deposits  of  the 
Potsdam,  the  Medina,  the  Chemung,  the  Catskill,  the  coarse  con¬ 
glomerates  of  the  Lower  Carboniferous,  or  the  calcareous  grit  of 
the  Schoharie  and  Oriskany,  all  show  different  conditions  of 
preservation. 

The  trilobites  in  the 
Potsdam  of  Wisconsin  and 
Minnesota  are  badly  broken 
up.  In  the  Cambrian  of 
Wisconsin  we  find  argillaceous 
layers  in  which  are  molds  and 
casts.  The  Medina  sandstone 
contains  poorly  preserved 
fossils,  and  shows  molds  of 
the  interior. 

In  the  calcareous  sandstones 
of  the  Chemung  we  find  well-preserved  organisms ;  but  in  the  Catskill 
sandstones  they  are  poorly  preserved. 

The  Utica,  Marcellus,  and  Genessee  slates  show  well-preserved 
originals;  but  the  majority  of  the  fossils  are  molds. 

1  Beecher,  Memoirs  0}  the  Peabody  Museum,  Yale  University,  Vol.  II,  Part  1. 


CONDITIONS  OF  FOSSILIZATION 


283 


The  articulate  Brachiopoda,  the  Anthozoa,  and  the  Bryozoa 
of  the  New  York  Hamilton  Shales  are.  well  preserved;  but  the 
Mollusca  occur  as  molds  and  casts.  Trilobites,  inarticulate  brachi¬ 
opods,  and  ostrocods  are  well  preserved.  In  the  Carboniferous 
shales  of  Illinois  the  Mollusca  are  well  preserved. 

In  the  Mesozoic  sandstones  of  the  West  the  fossils  are  casts 
and  molds  generally,  with  the  exception  of  the  Ostrea  and  allied 
genera. 

The  best  fossils,  as  a  rule,  are  found  in  those  limestones  which 
contain  more  or  less  argillaceous  or  siliceous  ingredients,  as  in  the 
Waldron  beds  of  Indiana,  the  Hamilton  layers  of  New  York,  and 
the  Lower  Carboniferous  of  Crawfordsville,  Ind, 

The  Schoharie  grit,  the  Oriskany  sandstone,  and  the  Calciferous 
beds  along  Lake  Champlain  give  siliceous  molds  on  weathering. 

The  Upper  Helderberg  limestone  gives  molds  and  casts  better  than 
sandy  deposits  in  general.  At  Cumberland,  Md.,  however,  the 
brachiopods  are  perfectly  preserved  as  casts  of  silica  in  the  Oriskany 
sandstone. 

In  the  Galena  limestone  many  of  the  fossils  are  preserved  as 
casts  composed  of  galena.  The  coal-measures  show  molds  coated 
with  pyrite.  In  the  Clinton  of  Oneida  County,  N.  Y.,  we  find 
limonite  casts.  In  the  Trenton  of  Wisconsin  and  Tennessee  we 
find  casts  of  silica.  In  the  Niagara  limestone  of  western  New  York 
we  find  calcite  casts;  but  on  the  weathered  surfaces  they  are  sili¬ 
ceous.  In  the  Schoharie  grit  we  find  siliceous  molds  of  the  interior 
of  brachiopods. 

In  general,  calcareous  skeletal  parts  show  an  unequal  persistence 
as  fossils  in  their  original  condition.  Chitinous  skeletons  are  never 
preserved  in  their  original  condition.  , 

GENERAL  CHARACTER  OF  INVERTEBRATE  SKELETONS 

c 

Chitin  is  confined  to  the  Arthropoda  and  a  few  brachiopods 
which  are  made  up  of  alternating  layers  of  phosphate  of  lime  and 
chitinous  material,  as  in  Lingula  anatina  and  in  the  graptolites. 

Silica  is  confined  to  the  arenaceous  Foraminifera,  the  Radio- 
laria,  the  Silicispongia,  and  Diatoms. 

Calcareous  material  is  confined  to  the  porcellaneous  and  vitreous 


284 


J.  CULVER  HARTZELL 


TABLE  I 

Horizon,  Locality,  and  Lithological  Character  of  Formations,  and  the 
Condition  of  Preservation  of  Classes,  Studied 


Condi- 

Horizon 

Locality 

Lithological 

Character 

Class 

tion  of 
Preser¬ 
vation 

Remarks 

Carboniferous 

Yellow  Cr., 

Shale 

Brachiopoda 

0  M* 

M  exterior  and  interior 

* 

Ohio 

Subcarbon* 

Crawfords- 

Limestone 

Spongiae 

C 

Pyrite 

iferous 

ville,  Ind. 

Mazon  Cr., 

Limestone 

Arachnide 

M  C 

Exterior 

Ill. 

Burlington, 

Limestone 

Anthozoa 

C 

Silica 

la. 

Crinoidea 

C 

Calcite,  silica 

Brachiopoda 

O  C 

C  siliceous 

(Waverly) 

Warren,  Pa. 

Sandstone 

Anthozoa 

C 

Exterior,  sandstone 

Crinoidea 

M 

Heads  and  arms,  sandstone 

Ophiuroidea 

M 

Exterior,  sandstone 

Brachiopoda 

OMC 

M  exterior  and  interior,  sil¬ 
ica;  exterior,  sandstone; 
interior,  sandstone,  calcite. 

C  exterior,  silica 

Lamellibran- 

O 

Slightly  changed 

chiata 

Gastropoda 

M 

Exterior,  sandstone 

Cephalopoda 

M  C 

Sandstone 

(Chester, 

Pulaski  Co., 

Limestone 

Bryozoa 

O 

Slightly  changed 

top  of) 

Ky. 

Limestone 

(St.  Louis 

Breckenridge 

Anthozoa 

C 

Silica 

Upper) 

Co.,  Ky. 

Limestone 

Brachiopoda 

O 

Slightly  changed 

Mt.  Vernon, 

Brachiopoda 

c 

Exterior,  silica 

Rockcastle 

Gastropoda 

c 

Silica 

Co.,  Ky. 

(St.  Louis) 

Taylor  Co., 

Limestone 

Anthozoa 

c 

Silica 

Ky. 

Brachiopoda 

c 

Silica 

Scaffold  Cone 

Limestone 

Anthozoa 

c 

Silica 

Ridge,  Mad¬ 
ison  Co.,  Ky. 

Brachiopoda 

c 

Silica 

(Keokuk) 

Russellville, 

Limestone 

Blastoidea 

c 

Silica 

Ky. 

Kings  Mt., 

Limestone 

Anthozoa 

0 

Spaces  filled  with  calcite 

Lincoln 

Co.,  Ky. 

Keokuk,  la. 

Shale 

Anthozoa 

0 

Spaces  filled  with  calcite 

Crinoidea 

c 

Silica 

Calcite 

Bryozoa 

M  C 

Calcite 

Brachiopoda 

c 

Exterior,  silica,  calcite 

Gastropoda 

c 

Calcite 

Trilobita 

c 

Exterior,  calcite 

(Burlington, 

Burlington,  la. 

Shale 

Crinoidea 

c 

Calcite 

Upper) 

Devonian, 

Livingston 

Slate 

Brachiopoda 

0  c 

O  slightly  carbonized 

Upper 

Co.,  N.  Y. 

C  exterior,  pyrite 

(Genesee) 

Pteropoda 

c 

Slate 

(Chemung) 

Cohocton, 

Sandstone 

Spongiae 

M  C 

M  exterior,  argillaceous  sand; 

N.  Y. 

C  sandstone 

Shale 

Anthozoa 

O 

Slightly  changed 

Loan  Valley, 

Crinoidea 

C 

Calcite 

N.  Y. 

Brachiopoda 

O 

Slightly  changed 

Devonian, 

Widder,  Can. 

Shale 

Anthozoa 

O 

Spaces  filled  with  calcite 

Middle 

Brachiopoda 

O 

Spiralia  pyrite;  in  some  cases 

(Hamilton) 

absent;  shell  filled _ with  cal¬ 
cite  and  at  times  with  mud 

Geneseo,  N.  Y. 

Shale 

Anthoza 

c 

Calcite;  spaces  filled  with 

same 

*0 — original;  M — mold;  C — cast. 


CONDITIONS  OF  FOSSILIZATION 


285 


TABLE  I. — Continued 


Horizon 

Locality 

Lithological 

Character 

Class 

Condi¬ 
tion  of 
Preser¬ 
vation 

Remarks 

Devonian, 

Crinoidea 

C 

Exterior,  calcareous 

,  Middle 

Bryozoa 

C 

Calcite 

(Hamilton) 

Brachiopoda 

OM  C 

M  and  C  calcite 

—Continued 

Lamellibran- 

O  C 

C  exterior 

chiata 

Gastropoda 

C 

Exterior  and  interior  in  mud; 

of  original  in  mud 

Cephalopoda 

C 

Calcite;  spaces  filled  with 

same 

Trilobita 

MC 

Exterior 

Pratts  Falls, 

Shale 

Brachiopoda 

OMC 

M  in  shale;  C  calcite 

N.  Y. 

Gastropoda 

C 

Calcite 

Cephalopoda 

C 

Calcite 

L.  Bethany, 

Shale 

Anthozoa 

C 

Calcite 

N.  Y. 

Brachiopoda 

O 

Slightly  changed 

Gastropoda 

OM 

O  slightly  changed;  M  of  ex- 

terior 

Michigan 

Limestone 

Hydrozoa 

C 

Replacement;  spaces  filled 

with  calcite 

Thunder  Bay, 

Limestone 

Anthozoa 

M  C 

Both  siliceous;  C  calcite 

Mich. 

Hydrozoa 

M  C 

M  siliceous;  C  calcite 

Crinoidea 

C 

Siliceous 

Bryozoa 

M  C 

M  siliceous;  C  calcite 

Brachiopoda 

OMC 

O  slightly  changed;  C  sili- 

ceous  and  calcite;  M  exte- 

rior  and  interior 

Cephalopoda 

C 

Siliceous 

Devonian, 

Columbus, 

Limestone 

Anthozoa 

C 

Calcite  and  spaces  filled  wi  th 

Lower 

Ohio 

same;  silica,  and  spaces 

(Cornifer- 

filled  with  same;  also 

ous) 

partly  calcite  and  silica; 

calcite  in  matrix,  but 

weather  out  silica 

Brachiopoda 

OMC 

M  and  C  calcite;  M  exterior, 

silica,  and  also  calcite 

Gastropoda 

M 

Interior,  calcite 

• 

Cephalopoda 

C 

Calcite 

Trilobita 

M 

Exterior,  silica 

Jeffersonville, 

Limestone 

Anthozoa 

C 

Calcite 

Ind. 

Crinoidea 

C 

Calcite 

Bryozoa 

C 

Calcareous 

Brachiopoda 

C 

Calcite,  also  calcareous 

Charleston, 

Limestone 

Anthozoa 

C 

Calcite;  Silica 

Ind. 

Crinoidea 

C 

Siliceous 

Brachiopoda 

C 

Siliceous 

Silurian, 

Albany  Co., 

Limestone 

Anthozoa 

C 

Silica 

Upper 

N.  Y. 

(Shaley) 

Bryozoa 

C 

Silica 

(Lower 

Brachiopoda 

C 

Silica 

Helderberg) 

Ostrocoda 

c 

Silica 

Trilobita 

c 

Silica 

Limestone 

Trilobita 

OMC 

O  slightly  changed;  M  inte- 

rior  calcareous,  exterior 

limestone;  exterior  and  in- 

terior  limestone;  also  clay; 

C  exterior  silica;  interior 

• 

silica 

Waterlime 

Anthozoa 

C 

Silica 

Limestone 

Brachiopoda 

OMC 

O  slightly  changed;  M  exter- 

ior  silica;  C  carbonized 

Pteropoda 

C 

Exterior,  limestone 

Merostomata 

M  C 

M  exterior;  shields  and  seg- 

ments  of  abdomen;  C  car- 

bonized 

Albany, 

Siliceous 

Anthozoa 

C 

Siliceous 

N.  Y. 

Limestone 

Bryozoa 

C 

Siliceous 

Brachiopoda 

C 

Siliceous 

286 


J.  CULVER  HARTZELL 


TABLE  I. — Continued 


Horizon 

Locality 

Lithological 

Character 

Class 

Condi¬ 
tion  of  _ 
Preser¬ 
vation 

Remarks 

Silurian, 

Jerusalem 

Water  lime 

Crinoidea 

O  C 

O  slightly  changed-,  C  stems, 

Upper 

Hill,  Her- 

Limestone 

calcite 

(Lower 

kimer  Co., 

Helderberg) 

N.  Y. 

—Continued 

Cedarville, 

Limestone 

Anthozoa 

C 

Calcite;  silica 

Herkimer 

Brachiopoda 

0  C 

C  calcareous;  siliceous 

Co.,  N.  Y. 

Gastropoda 

C 

Exterior,  calcareous 

Trilobita 

0 

Tests  of  pygidium  slightly 

changed 

Indian  Ladder, 

Limestone 

Spongiae 

C 

Siliceous 

Albany  Co., 

Calcareous 

N.  Y. 

Anthozoa 

C 

Siliceous 

Crinoidea 

C 

Ring  formed  of  silica  and 

filled  with  calcite 

Bryozoa 

C 

Siliceous 

Brachiopoda 

C 

Siliceous 

(Niagara) 

Lock  port, 

Anthozoa 

C 

Silica;  calcite 

N.  Y. 

Crinoidea 

C 

Calcite 

Brachiopoda 

O 

Slightly  changed 

Crustacea 

O 

Tests;  slightly  changed 

Charleston, 

Siliceous 

Anthozoa 

C 

Calcite;  silica 

Ind. 

Limestone 

Crinoidea 

C 

Siliceous 

Brachiopoda 

C 

Siliceous 

Silurian, 

New  York 

Shale 

Graptolitoidea 

C 

Carbonized 

Lower 

Cincinnati,  0. 

Limestone 

Anthozoa 

C 

Calcite 

(Hudson 

Brachiopoda 

O 

Slightly  changed 

River) 

Cephalopoda 

c 

Calcite 

(Lower 

Clarksville, 

Shale 

Spongiae 

M  C 

M  arenaceous;  pyrite;  man- 

Hudson) 

N.  Y. 

ganese;  C  spicules  re- 

placed  by  silica  and  canals 

filled  with  calcite 

Franklin 

Shale 

Spongiae 

C 

Spicules  and  walls  of  canals 

Co.,  Ky. 

of  silica;  also  calcite;  ca- 

nals  filled  with  chert 

(Trenton) 

Kentucky 

Limestone 

Spongiae 

C 

Silica 

Foraminifera,  the  Coelenterates,  except  the  Silicispongia,  the  Echino- 
dermata,  some  of  the  Vermes,  the  Molluscoidea,  and  the  Mollusca. 

Chitin  undergoes  more  or  less  alteration.  In  some  cases  it  is 
replaced  by  calcite. 

Silica  secreted  by  organisms  is  dissolved  with  comparative 
ease.  It  is  at  times  replaced  by  calcite.  The  siliceous  sponges 
are  very  commonly  replaced  by  calcite.  If  a  siliceous  organism 
be  found  as  a  siliceous  fossil,  the  original  silica  has  probably  been 
either  altered  or  replaced  by  silica. 

Carbonate  oj  lime  is  easily  dissolved.  It  is  made  use  of  in  two 
forms  by  organisms.  In  the  form  of  calcite  it  is  more  durable  than 
in  the  form  of  aragonite.  This  is  due  to  the  differences  in  com- 
pactibility,  hardness,  and  specific  gravity.  Gastropods,  many  lamel- 
libranchs,  corals,  and  other  organisms  composed  of  aragonite  crumble 
down  and  pass  into  calcite,  or  disappear,  while  many  composed 


CONDITIONS  OF  FOSSILIZATION 


287 


of  calcite  may  remain.  In  some  strata  the  aragonite  skeletons 
have  entirely  disappeared.  This  is  most  likely  to  occur  in  pervious 
beds.  The  presence  of  calcite  forms  does  not  necessarily  imply 
that  they  were  not  associated  with  aragonite  forms.  The  conditions 
of  preservation  also  vary.  In  the  Mesozoic  clays  we  find  cephalo- 
pods  as  originals,  while  in  the  Palaeozoic  clays  they  are  calcite  casts. 
Mytilus  edulis  secretes  aragonite  as  its  inner  layer  and  calcite  as 
its  outer  layer.  Fossils  occur  in  which  the  inner  layer  is  gone.  Cah 
cite  replaces  aragonite  at  times;  but  in  such  cases  the  internal 
organic  structure  is  gone.  As  yet  no  example  of  aragonite  replacing 
calcite  has  been  reported. 

Under  Table  I  is  given  the  horizon,  locality,  and  lithological 
character  of  the  formations  studied,  and  also  the  class,  conditions 
of  preservation,  and  remarks  in  connection  with  certain  forms  found 
in  these  formations. 

GENERAL  MINERAL  CHARACTER  OF  LIVING  INVERTEBRATES 

Foraminijera. — The  vitreous  and  porcellanous  forms  are  calcite. 
The  arenaceous  forms  are  siliceous  throughout,  or  have  a  sandy- 
siliceous  layer  incrusting  an  interior  calcareous  layer.  The  Gro- 
midae  are  chitinous. 

Radiolaria. — Some  are  composed  of  acanthine  and  some  of  silica. 

Spongiae. — The  Myxospongiae  are  composed  entirely  of  soft 
tissues.  The  Cerato spongiae  are  made  up  of  spongin  fibers.  The 
Silicispongiae  are  made  up  of  siliceous  elements  or  contain  siliceous 
spicules.  The  Calcispongiae  contain  calcareous  spicules. 

Anthozoa. — The  Madreporaria  are  aragonite,  and  the  Alcyo- 
naria  are  calcite. 

Hydrozoa. — The  Hydrocorallinae  are  calcite  (?)  and  the  Tubu- 
lariae  calcite  (?)  and  chitin.  The  Graptolitoidea  are  chitin. 

Echinodermata. — Calcite. 

Vermes. — Calcite  (  ?). 

Bryozoa. — Calcite  and  aragonite  (?). 

Brachiopoda. — Calcite. 

Lamellibranchiata. — Some  are  calcite,  some  aragonite,  and  some 
both  calcite  and  aragonite  in  layers. 

Scaphopoda. — Aragonite  (?). 


288 


J.  CULVER  HARTZELL 


Gastropoda. — Aragonite.  Some  are  composed  of  aragonite  and 
calcite. 

Cephalopoda. — Mainly  aragonite.  Nautilus  pompilius  has  cal¬ 
cite  for  its  inner  layer  and  septum,  instead  of  aragonite  as  hereto¬ 
fore  reported. 

Crustacea. — Mainly  calcite. 

REPLACING  MINERALS 

The  hard  parts  of  invertebrate  organisms  are  composed  of  more 
or  less  soluble  mineral  matter,  and  are  often  replaced  by  other  min¬ 
erals  which  fill  the  cavities  left  by  the  hard  parts.  There  may  be 
molecular  replacement  as  the  original  gradually  disappears,  or  the 
cavity  may  be  filled  by  precipitation  after  the  original  has  entirely 
disappeared.  Chemical  reaction  may  take  place,  producing  new 
minerals  as  the  elements  in  the  original  unite  with  the  elements  in 
the  matrix,  or  elements  brought  in  due  to  the  porosity  of  the  imbed¬ 
ding  material. 

The  imbedding  material  always  contains  minerals  that  are  easily 
dissolved  under  such  conditions  as  heat,  pressure,  and  moisture, 
and  they  may  be  deposited  separately  or  in  combination.  The 
predominating  mineral  is  apt  to  be  found  forming  molds  or  casts 
of  the  lost  parts. 

In  calcareous  shales  we  find  calcite  casts.  In  siliceous  lime¬ 
stones  we  find  siliceous  casts.  In  ferruginous  formations  we  find 
siderite,  pyrite,  limonite,  etc.,  casts  and  molds.  In  galena-bearing 
formations  we  find  casts  composed  of  that  sulphide.  These  illus¬ 
trations  might  be  extended;  but  they  suffice  to  show  how  the  char¬ 
acter  of  a  formation  affects  an  original  skeletal  part  in  its  preservation. 

The  most  common  replacing  minerals  are  calcite,  pyrite,  silica, 
limonite,  sphalerite,  vivianite,  barite,  malachite,  siderite,  and  hem¬ 
atite.  The  list  of  replacing  minerals  is  quite  large,  thirty-five  being 
the  number.  Others  undoubtedly  occur,  and  sooner  or  later  will 
be  added  to  our  present  list.  Under  Table  II  is  given  the  replacing 
minerals  found,  and  their  symbols,  Dana’s  system  being  followed 
in  their  classification. 

In  the  paper  to  follow  will  be  given  a  table  showing  the  mineral 
composition  of  the  more  closely  related  living  and  fossil  forms  studied. 


CONDITIONS  OF  FOSSILIZATION 


289 


TABLE  II 

Minerals  Replacing  Minerals  Secreted  by  Invertebrates1 

Carbonates,  Anhydrous:  Calcite  (CaC03),  Cerussite  (PbC03),  Mag¬ 
nesite  (MgCQ3),  Siderite  (FeCO,),  Smithsonite  (ZnC03). 

Carbonates,  Basic  Hydrous:  Malachite  (CuC03.Cu(OH)2). 

Chlorides,  Anhydrous:  Cerargyrite  (AgCl). 

Fluorides,  Anhydrous:  Fluorite  (CaF2). 

Metals:  Copper  (Cu),  Silver  (Ag). 

Non-metals:  Sulphur  (S). 

Oxides,  Anhydrous:  Cassiterite  (Sn02). 

Oxides,  Hydrous:  Limonite  (2Fe203 .3H20),  Psilomelane  (H4MnOs  (?). 
Oxides,  Sesqui:  Hematite  (Fe203). 

Phosphates,  Anhydrous:  Apatite  ((CaF)Ca4P30I2). 

Phosphates,  Hydrous;  Vivianite  (Fe3P2Os). 

Sulphates,  Anhydrous:  Barite  (BaS04),  Celestite  (SrS04),  Anglesite  (PbS04). 
Sulphates,  Hydrous:  Gypsum  (CaS04. 2H20). 

Sulphides,  Di:  Pyrite  (FeS2),  Marcasite  (FeS2). 

Sulphides,  Mono:  Sphalerite  (ZnS),  Galena  (PbS),  Chalcocite  (Cu2S),  Cin¬ 
nabar  (HgS). 

Silicates,  Hydrous:  Kaolinite  (H4Al2Si204),  Giimbelite  (Si02.Al203.Fe2- 
03.Mg0.K20.Na20.H20),  Glauconite  (Hydrous  silicate  of  Fe  and 
K),  Margarite  (H2CaCl4Si20I2). 

Silicates,  Sub:  Calamine  (H2Zn2SiOs). 

Silicon,  Oxides  of:  Flint  (Si02),  Silica  (Sio2),  Sand  (Si02). 

REFERENCES  TABLE  II 

A 

Bernhard,  Elements  de  Paleontologie,  1895. 

Geikie,  Text-Book  of  Geology ,  3d  ed.,  1893. 

Gratacap,  American  Naturalist ,  Vol.  XXXI,  No  363. 

Nicholson  and  Lydekker,  Manual  of  Paleontology,  3d  ed.,  Vol.  I,  1889. 
Reis,  “Ueber  Petrifikation  der  Muskel,”  Archiv  fur  mikroskopische  Anatomic, 
Vol.  XLI. 

Trabucco,  La  Petrificazione,  1887. 

Von  Zittel,  Handbuch  der  Paldontologie. 

White,  Bulletin  of  the  United  States  Geological  Survey,  Vol.  I,  No.  I,  Art.  8,  1879. 

- Bulletin  of  the  United  States  Geological  and  Geographical  Survey  of 

the  Territories,  Vol.  V,  1880. 

1  The  writer  is  indebted  to  the  late  Professor  Beecher  for  valuable  material  from 
his  private  collection;  to  Rt.  Rev.  Joseph  C.  Hartzell,  Bishop  of  Africa,  for  material 
from  that  continent;  to  Rev.  Morton  Culver  Hartzell,  for  material  from  Germany; 
and  to  Major  Ebenezer  Thresher,  for  material  from  France. 


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